Thin film magnetic recording heads have gained wide acceptance in the data storage industry. Due to their small size, thin film magnetic heads are ideal for recording data onto narrow tracks, thereby increasing the total data storage capacity of a magnetic medium such as a disk or a tape.
Typically, a thin film magnetic head is produced by laminating one or more layers of thin magnetic film onto a substrate. These films are usually on the order of 2-4 microns thick, although thinner and thicker films are sometimes produced. During the manufacturing process, a substrate (made of Ni--Zn ferrite or Alumina, for example) is precision machined to desired specifications. Alternating layers of conductive and insulating materials are then deposited onto the substrate by means of evaporation, sputtering, plating, or some other deposition means, which allows for precise control of the deposition thickness. Etching, ion milling or other means are used to shape and form the deposited layers so that the desired configuration of each layer is provided. For details relating to the general manufacturing process used to produce thin film magnetic recording heads see a book entitled Handbook of Thin Film Deposition Processes & Techniques: Principles, Methods, Equipment & Applications edited by Klaus Schuegraf, and published in 1989 by Noyes Press, which is hereby incorporated by reference.
A completed magnetic head typically includes two conductive layers, called poles, separated by an insulating layer. The poles are conductively connected at one end so that the overall configuration of the poles is annular, somewhat like that of a horseshoe. Conductive wires or strips are embedded within the insulating layer between the poles. The insulating layer is typically narrowest (e.g., on the order of 0.5 microns) near the tips of the poles (i.e., at the end where the poles are not conductively connected). The portion of the insulating layer near the pole tips is called the front gap (or, as often referred to hereinafter, simply the gap), and the thickness of the insulating layer between the pole tips is called the gap length. Typically, the substrate on which the layers have been deposited is formed into a slider defining one or more air bearing surfaces. The portion of the insulating layer near the region where the poles are conductively connected is called the rear gap. Once the entire head has been completed, the head is mounted onto a suspension assembly, which supports the head in a desired position relative a magnetic medium.
During write operation of the head, electrical current flows through the conductive strips, which induces a magnetic field or flux within the poles. The magnetic flux flows in a circuit through the poles, but is impeded by the discontinuity at the gap. Due to the discontinuity at the gap, the magnetic field protrudes out into the regions proximate to the gap. It is this protruding portion of the magnetic field that is used to record data onto magnetic media.
A magnetic medium typically includes magnetic dipoles, which may be oriented by means of a sufficiently strong magnetic field. As the magnetic medium (e.g., a tape or disk) moves past the pole tips of the recording head, flux protruding from the gap region interferes with the magnetic medium. In this way, variations in the magnetic flux cause variations in dipole orientation along the length of-the magnetic medium. These variations in the magnetic medium constitute retrievable information.
In applications where it is desirable to store digital information, the information recorded onto a magnetic medium is designated as either a "1" or a "0". To record a bit as a one, a positive magnetic flux pulse might be induced, which causes the dipoles to orient in one direction (thereby creating a "positive" magnetic bias in the medium). To record a bit as a zero, a negative flux pulse causes the dipoles to orient in the opposite direction (thereby creating a "negative" magnetic bias in the medium). In high density magnetic storage, it is desirable to generate flux pulses that have a high gradient (that is, pulses that are very steep) so that many pulses can be recorded in close proximity to one another. It is also desirable to produce pulses with an amplitude significantly higher than the expected background noise level, so that a high signal-to-noise ratio (SNR) is obtained. A high amplitude pulse is also desirable because newly recorded data is usually written over old data on a magnetic medium. The technique of writing new data over old data is commonly referred to as "overwrite."
Once a sufficiently strong magnetic bias has been imparted to the magnetic medium by a write head, the information encoded into the magnetic medium may be retrieved by moving the medium past the gap so that magnetic flux produced by the motion of the magnetic medium induces current to flow in the coil windings. The current produced by the motion of the medium past the head typically takes the form of electrical pulses. The amplitude of these pulses is detected within a predetermined time period, or window, and, depending upon the detected amplitude of the pulse, the pulse is designated as a digital 1 or 0.
In current high-frequency, high-density, recording systems, timing is critical since only a small time window (e.g., 15 nanoseconds) is allotted for the detection of a pulse. If a pulse is shifted in time or distorted in amplitude, the pulse may not be detected within the allotted window or may be distorted to such a degree that the amplitude of the pulse is ambiguous. Thus, it is essential that the morphology of the data pulse be accurately detected to prevent time shifts or amplitude distortions.
FIG. 1 shows a plot of magnetic field strength of an exemplary positive flux pulse 20 versus time, observed when a magnetic tape or other magnetic medium passes near the recording head gap. As shown in FIG. 1, the pulse 20 has a main, high amplitude component 25, as well as leading and trailing negative undershoot components 30, 32. The undershoot components 30, 32 are produced when the edges of the leading and trailing poles of the recording head detect fringe fields from the magnetic medium, while the main high amplitude component is produced by the strong magnetic field present adjacent to the gap. It has been found that the undershoot components tend to distort the amplitude of the main read pulse 25, thereby contributing to background noise. In addition, the leading and trailing undershoots 30, 32 play a major role in peak-shift (that is, a time shifting of the main high amplitude component of the detected data pulse). Background noise interference and peak-shift may severely compromise the quality of detected data. Thus, it is important to minimize the effects due to these undershoots.
The physical geometry of the recording head may be altered to produce changes in the morphology of the flux pulse detected by the recording head. However, current methods of producing magnetic recording heads have several serious limitations with respect to their ability to control the physical geometry of the recording heads. Furthermore, the tolerances achievable by current techniques are unacceptable for purposes of accurately shaping the recording heads to a desired configuration in many applications.